1. Field of the Invention
The present general inventive concept relates to a multi-voltage power supply, and more particularly, to a multi-voltage power supply to independently control multiple voltages using a simple structure.
2. Description of the Related Art
In general, devices, such as personal computers (PCs), printers, photocopiers, monitors, and communication terminals, require a heavy-duty power supply system having a simple structure, a small size, and consistent power supply capability. Current source type power supplies are generally used for this required heavy-duty power supply system.
FIG. 1 is a circuit diagram illustrating the basic operation of a current source type power supply called a “flyback converter,” which is a type of direct current (DC)/DC converter.
Referring to FIG. 1, the flyback converter includes a transformer T having a predetermined winding ratio, a primary circuit 10 connected to the primary coil, i.e., the input coil, of the transformer T, and a secondary circuit 20 connected to the secondary coil, i.e., the output coil, of the transformer T. The primary circuit 10 and the secondary circuit 20 are isolated from each other by the transformer T.
The primary circuit 10 includes a control switch S connected in series between the primary coil of the transformer T and ground. The control switch S controls a stored energy or a transfer operation of the transformer T by switching an input voltage in response to a control signal input from an output voltage controller 30.
The secondary circuit 20 includes a rectifier 21 which is used to rectify a current transferred from the transformer T. The rectifier 21 includes a diode D and a capacitor C, which are connected in series, and together are connected in parallel to the secondary coil of the transformer T. An output terminal is formed at the both ends of the capacitor C. That is, an external load can be connected in parallel to the capacitor C. The secondary circuit 20 also can include a filter (not illustrated) for filtering high frequency noise and Electro Magnetic Interference (EMI) and an output voltage control circuit (not illustrated).
If the control switch S included in the primary circuit 10 is in an ON state (closed), a voltage having a polarity opposite to that of the primary coil of the transformer T is induced in the secondary coil, resulting in an inverse bias state of the diode D of the rectifier 21. Thus, a current flowing through the secondary circuit 20 is blocked, and energy is stored in the form of a magnetization inductance of the transformer T. That is, in an ON state (closed) of the control switch S, a current transfer by the transformer T is not performed, and the entire energy supplied to the primary coil is stored in the form of the magnetization inductance of the transformer T.
If the control switch S is in an OFF state (open), a voltage, having a polarity opposite to when the control switch S is in the OFF state (open), is induced in the secondary coil of the transformer T, resulting in an ON state of the diode D of the secondary circuit 20. Thus, a current due to the magnetization inductance of the transformer T is transferred to the secondary circuit 20, and a DC voltage rectified by the rectifier 21 is thereby output through the output terminal.
The output voltage controller 30 is connected to the output terminal of the secondary circuit 20. The output voltage controller 30 applies a control signal to the control switch S by feeding back the output voltage of the secondary circuit 20. The control signal acts as a signal for controlling a duty rate of the control switch S. Thus, the output voltage can be controlled by controlling the operation of the control switch S.
As described above, by using the magnetization inductance component of the transformer T as a boost inductor, the flyback converter stores energy in the form of the magnetization inductance of the transformer T when the control switch S included in the primary circuit 10 is in the OFF state, and supplies the rectified DC voltage by transferring the current, due to the change in magnetization inductance, to the secondary coil of the transformer T when the control switch S is in the ON state.
Thus, in terms of the secondary circuit 20, since the transformer T acts as a current source for supplying a current periodically, each power supply having this principle is called a current source type power supply. Various other types of current source type power supplies, besides the flyback converter, exist, according to various circuit configurations added to a primary circuit.
Since such a current source type power supply has a secondary circuit with a simpler rectifier structure and less components compared to other type power supplies, the current source type power supply has an advantage when used to output multiple voltages. That is, since secondary circuits corresponding to the multiple voltages must be included, if the secondary circuits have a simple structure, a total size of a power supply can be reduced.
Due to this advantage, various current source type multi-voltage power supplies have been suggested. However, since conventional current source type multi-voltage power supplies use a plurality of transformers and a plurality of inefficient regulator chips to control output voltages of secondary circuits, and/or have a complex structure in which an output voltage feedback circuit of each secondary circuit is connected to a primary circuit, the advantage of the current source type power supplies cannot be properly utilized.